Apparatus and method of cryogenic cooling for high-energy cutting operations

ABSTRACT

A cryogenic fluid jet is used in an apparatus and a method for remote cooling of a cutting tool engaged in machining a workpiece under high-energy conditions, such as high-speed machining, hard-turning, cutting of difficult to machine materials, and combinations thereof. The apparatus and method use a stabilized, free-expanding cryogenic fluid jet having a pulse cycle time less than or equal to about 10 seconds. The apparatus and method increase the cleanliness of machined parts and chips and machining productivity of hard but brittle tools, including but not limited to tools which should not be cooled with conventional cooling fluids.

BACKGROUND OF THE INVENTION

The present invention relates to the field of machining of material bycutting (e.g., shaping parts by removing excess material in the form ofchips), and more particularly machining of materials by cutting withcryogenically cooled cutting tools.

Numerous references are cited throughout this application, including theendnotes which appear after the Detailed Description of the Invention.Each of those references are incorporated herein by reference withregard to the pertinent portions of the references cited herein.

As used herein, the term “cutting” includes but is not limited to thefollowing operations: turning, boring, parting, grooving, facing,planing, milling, drilling, and other operations which generatecontinuous chips or fragmented or segmented chips. The term cutting doesnot include: grinding, electro-discharge machining, ultrasonic cutting,or high-pressure jet erosion cutting, i.e., operations generating veryfine chips that are not well defined in shape, e.g., dust or powder.

Cutting hard or difficult to machine materials, as well as high-speedcutting of materials from all groups except the low-melting point groupincluding zinc or polymers, leads to very high levels of energydissipated at the cutting tool. Table 1 below presents examples of easyand difficult to machine ferrous and non-ferrous metals with theirmachining responses modified by both composition and thermo-mechanicalcondition. Materials characterized by the unit power (P_(c)) of morethan 1 hp/in³/minute, unit energy (E_(c)) of more than 2.7 J/mm³, and/orhardness of more than 30 HRC are considered difficult to machine. In thecase of steels and other metals melting above 1400° C., high-speedmachining proves difficult even if the hardness level is only 25 HRC.TABLE 1 Examples of Hardness, Power, Energy and Temperature Encounteredin Cutting⁽¹⁾ Nominal increase in Assumed Work Specific AssumedMaterial/Chip Unit Power Unit Energy Density Specific Heat TemperatureMaterials: Hardness: [hp/in³/minute] [Joules/mm³] [grams/cm³][cal./(gram * K)] [deg. K or C.] Magnesium  40-90 HB 0.13-0.17 0.36-0.46Low strength aluminum alloys  30-150 HB 0.20 0.55 2.7 0.21 230 6061 - T6aluminum alloy 0.35 0.96 2.7 0.21 400 2024 - T4 aluminum alloy 0.46 1.262.7 0.21 520 Soft copper alloys  10-80 HRB 0.50 1.37 8.9 0.09 40070Cu-30Zn brass 0.59 1.61 Copper and harder copper alloys  80-100 HB0.70-0.80 1.91-2.18 8.9 0.09 580 Steels: AISI 1020 carbon steel 150-175HB 0.58 1.58 7.8 0.11 440 AISI 1020 carbon steel 176-200 HB 0.67 1.837.8 0.11 500 Carbon, alloy, and tool  35-40 HRC 1.15 3.14 7.8 0.11 870steels,  40-50 HRC 1.20 3.28 7.8 0.11 900 various hardness  50-55 HRC1.60 4.37 7.8 0.11 ˜1200 levels . . .  55-58 HRC 2.75 7.51 7.80.11 >1500 Stainless steels, wrought and 135-275 HB 1.05 2.87 cast, 30-45 HRC 1.12 3.06 various hardness 150-450 HB 1.12 3.06 levels: . . .Precipitation hardening stainless steels Soft grades of cast irons110-190 HB 0.55 1.50 Gray, ductile, and malleable 190-320 HB 1.12 3.06grades Titanium alloys 250-375 HB 1.0-1.9 2.73-5.18 4.4 0.12 1186->1600Nickel based superalloys 200-360 HB 2.0 5.46 8.9 0.11 >1350 Niobiumalloys 217 HB 1.4 3.82 Molybdenum 230 HB 1.6 4.37 10.2 0.06 1710Tantalum 210 HB 2.25 6.14 Tungsten 320 HB 2.3 6.28 19.2 0.03 2440Notes:1. Unit power - power at cutting tool required to remove work materialat the rate of 1 in³/minute.2. Unit energy - total energy dissipated by cutting tool removing 1 mm³of material. 1.0 hp/in³/min = 2.73 J/mm³.3. Listed above, average values of unit power required in turning arevalid for sharp high-speed steel (HSS) and carbide (WC-Co) tools cuttingwithin the feedrate range of 0.005 to 0.020 inches per revolution andexclude spindle efficiency factor. Average values of unit power requiredin milling may vary by +/− 10%.4. Values of unit power should be multiplied by a factor of about 1.25in the case of cutting with dull tools or tools characterized by anegative rake geometry.5. Calculated above, nominal increase in chip temperature is an estimateassuming: (1) constant specific heat of work material across the entiretemperature range, (2) no energy losses to work material and tool, and(3) a uniform temperature distribution across chip thickness includingthe chip/tool contact interface within so-called secondary shear zone.

Table 1also shows how the unit power and energy translate into hightemperatures of a machined chip staying in contact with the cuttingtool. It is clear that the high-energy materials and cutting conditionsrequire tool grades retaining hardness at the highest temperatures—hardbut brittle grades of cemented carbides (WC—Co) and, ideally, advancednon-metallic tool materials that offer an ultimate level of hardness atthe cost of low rupture strength and fracture toughness.

Table 2 below outlines the typical values of traverse rupture strength(TRS) and fracture toughness (K_(1c)) of the major groups of toolmaterials. TABLE 2 Selected Properties of HSS, Carbide and Advanced ToolMaterials - Cermets, Ceramics and Diamond⁽²⁾ Traverse Fracture rupturetoughness Tool material strength (MPa) (K_(1c)) MPa m^(1/2) Al₂O₃500-700 2.5-4.5 Al₂O₃—TiC 600-850 3.5-4.5 Al₂O₃-1% ZrO₂ 700-900 5-8Al₂O₃—SiC 550-750 4.5-8   SiAlON 700-900 4.5-6.5 Si₃N₄  100-1000 1.5-8  SiC 550-860 4.6 Polycryst. CBN (PCBN)  800-1100   4-6.5 Polycryst.Diamond (PCD)  390-1550 6-8 TiC—TiN—WC—TaC—Ni—Co—Mo 1360 8.5(C7-C8/C3-C4 class) 97WC-3Co (with alloying additions) 1590 971WC-12.5TiC-12TaC-4.5Co 1380 84WC-16Co (straight cemented 3380  10-13.5 carbide grades) High speed steel M42 (CPM grade) 4000

Comparing to the traditional high-speed steel (HSS) and tougher gradesof cemented carbides containing more cobalt binder, the advanced,non-metallic tool materials are significantly more brittle, i.e.,sensitive to irregularities in stress loading, irregularities in thermalloading or cooling and thermal stress shocking. Tools with a TRS valueof less than 3 GPa (3000 MPa) and a K_(1c) value of less than 10 MPam^(1/2) are considered brittle and prone to rapid fracturing underhigh-energy cutting conditions. Thus, the machining community is awareof the necessity of either avoiding the use of conventional cuttingfluids when machining with these brittle tool materials or, if it ispossible and practical in a given cutting operation, using the brittletool materials with extreme care by a complete and uniform flooding ofthe tool, chip, and contact zone.

For example, numerous publications and tool manufacturer recommendationsalert machining operators to the problem of reduced life of ceramictools on contact with conventional cutting fluids. Despite the inherentdeficiencies, e.g., overheated workpiece, reduced dimensional accuracy,or risk of chip fires, dry machining is recommended if hard but brittletools are used. P. K. Mehrotra of Kennametal teaches in “Applications ofCeramic Cutting Tools”, Key Engineering Materials, Vol. 138-140 (1998),Chapter 1, pp. 1-24: “the use of coolants is not recommended when thesetools are used to machine steels due to their low thermal shockresistance”. R. C. Dewes and D. K. Aspinwall (“The Use of High SpeedMachining for the Manufacture of Hardened Steel Dies”, Trans. ofNAMRI/SME, Vol. XXIV, 1996, pp.21-26) tested a range of oxide andnitride tools including: 71% Al₂O₃—TiC, 75% Al₂O₃—SiC_(w), 50%CBN—AlB₂—AlN, 50%-TiC—WC—AlN—AlB₂, 80% CBN—TiC—WC, as well as 95%CBN—Ni/Co. They found that the use of a conventional cooling fluidapplied by flooding or spraying resulted in the reduction of tool lifeby more than 95% except for the whisker reinforced alumina, for whichthe life was shortened by about 88%. Similar test results showing adramatic tool failure by bnittle chipping on contact with cooling fluidhave been published for PCBN cutting inserts by T. J. Broskea et al. ofGE Superabrasives at MMS Online (www.mmsonline.com/articles) and byothers elsewhere.

Table 3 below represents typical machining conditions recommended in theprior art 15 for a range of work materials and tool materials. Whiledifferent combinations of depth of cut (DOC), feedrate (F), cuttingspeed (Vc), and unit power (Pc), lead to high or low total power levels(P), the most important value characterizing high-energy cutting andcritical to tool life is the power flux (P_(f)), which is calculated bydividing P by the cross-sectional area of an undeformed chip (a productof DOC and F). TABLE 3 EXAMPLES OF MACHINING CONDITIONS RECOMMENDED INPRIOR ART FOR A RANGE OF CUTTING, VARIABLES, INCLUDING WORK MATERIALS,WORK HARDNESS LEVELS, AND TOOL MATERIALS Depth Recommended Work MaterialAssumed: Unit Power Work of cut, Feedrate, Cutting Speed, Removal Powerin Total Flux. Material Tool Type and DOC F Medium Value, Rate, MRRCutting, Pc Power, Pf [hp/ Work Material Hardness Material [inches][inch/rev] Vc [feet/min] [in3/min] in3/min] P [hp] [kW/mm2] CarbonSteel, 150 HB indexable carbide, 0.150 0.020 490 17.6 0.6 10.2 3.9 1020grade C-6 (P20) Carbon Steel, 150 HB HSS, M2-M3 0.150 0.015 120 3.2 0.61.9 1.0 1020 grade H13 Tool Steels, 48-50 HRC indexable carbide, 0.1500.010 150 2.7 1.2 3.2 2.5 Q&T C-8 (P01) H13 Tool Steels, 48-50 HRCindexable carbide, 0.300 0.015 120 6.5 1.2 7.8 2.0 Q&T C-8 (P01)High-carbon Alloy 52-54 HRC indexable carbide, 0.150 0.005 115 1.0 1.61.7 2.6 of Toot Steels C-8 (P01) Cold Work Too 58-60 HRC PCBN (DBC50)0.012 0.004 490 0.3 3.0 0.8 20.4 Steel Austenitic St. 135-185 HBindexable carbide, 0.150 0.020 350 12.6 0.8 10.1 3.9 Steels C-2(K10/M10) Austenitic St. 135-185 HB Cold-pressed 0.150 0.010 900 16.20.8 13.0 10.0 Steels Alumina, ceramic Austenitic St. cold drawnindexable carbide, 0.150 0.015 300 8.1 0.9 7.3 3.7 Steels to 275 HB C-3Austenitic St. cold drawn HSS, T15-M42 0.150 0.015 80 2.2 0.9 1.9 1.0Steels to 275 HB Ti-6Al-4V ELI 310-350 HB indexable carbide, 0.150 0.008195 2.8 1.4 3.9 3.8 C-2 (K10, M10) Ti-6Al-4V ELI 310-350 HB HSS, T15-M420.150 0.010 60 1.1 1.4 1.5 1.2NOTES:CUTTING POWER, POWER FLUX, AND VELOCITY INDEX ARE ESTIMATED FROM DATA INTABLE 1.REFERENCES FOR MACHINING CONDITIONS - IAMS AND ASM LISTED IN TABLE 1.Power Flux = Total Power/DOC/F1 hp/in2 = 1.15 W/mm2

The representative examples in Table 3 are not intended to be anexhaustive list. Persons skilled in the art will recognize that numerousother conditions are possible that would result in similar patterns.

High values of power flux indicate the magnitude of potential upset inthermo-mechanical tool loading or irregularity in tool cooling. Only theHSS tools and certain cemented carbide tools operate under the range ofcutting conditions where these process irregularities can be neglected.Being a product of cutting speed and unit power, power flux indicateswhether a given set of machining conditions leads to a high-energycutting situation. If a cutting speed is selected for a given tool,depth of cut, and feedrate, which is higher than the cutting speedrecommended by the tool manufacturer, and/or the work material requiresunit cutting power exceeding 1 hp/in³/minute, the resultant power fluxvalue exceeds the conventional power flux value and the operation may beclassified as high-energy cutting.

Although the machining industry has strong economic incentives toenhance cutting operations within the high-energy range, it is limitedby tool overheating, high power flux values, and inability of removingcutting energy from tools in a uniform manner required to prevent rapidfailures. All tool materials, including HSS, carbides, and refractoryceramics, have one thing in common—as the temperature of the toolmaterial increases, the tool material softens and may develop localized,internal stresses (due to thermal expansion, especially if compoundedwith limited conductivity), as described by E. M. Trent and P. K. Wrightin “Metal Cutting”, 4th Ed., Butterworth, Boston, Oxford, 2000, and theASM Handbook on “Machining, Ceramic Materials”. This poses limits onworkpiece hardness, cutting speed, and power flux during machining. Withconventional machining methods, the industry is unable to cope with thecooling problem while satisfying the other needs enumerated above. Otherproblems facing the machining industry include significant environmentaland health related problems associated with the conventional cuttingfluids and coolants presently used in the industry. For example, carbondioxide (CO₂), a commonly used industrial coolant, is a greenhousegenerator. Also, since CO₂ is denser than air it presents a potentialasphyxiation concern. In addition, CO₂ also has the potential to causeacid corrosion, since it is soluble in water. Freons and freonsubstitutes, some other commonly used coolants, also are greenhousegenerators and ozone depleters. These substances also are explosiveand/or toxic when heated on contact with red-hot solids. Other coolantswhich can be explosive include hydrocarbon gases and liquified ammonia.Coolants such as cryogenic/liquified air with oxygen in it can result inchip fires.

There exists a relatively large body of prior art publicationspertaining to cryogenic cooling of tools, including: WO 99/60079 (Hong)and U.S. Pat. No. 5,761,974 (Wang, et al.), U.S. Pat. No. 5,901,623(Hong), U.S. Pat. No. 3,971,114 (Dudley), U.S. Pat. No. 5,103,701(Lundin, et al.), U.S. Pat. No. 6,200,198 (Ukai, et al.), U.S. Pat. No.5,509,335 (Emerson), and U.S. Pat. No. 4,829,859 (Yankoff). However,none these publications nor the other prior art references discussedherein solve the problems discussed above or satisfy the needs set forthbelow.

U.S. Pat. No. 5,761,974 (Wang et al.) discloses a cryogenically cooledcap-like reservoir placed at the top of a cutting tool, as shown in FIG.1A herein (corresponding to FIG. 1 of Wang et al.). Wang's method andapparatus provides for uniform and stable cooling, except that thereservoir requires dedicated tooling and repositioning if depth of cutand/or feedrate are changed during cutting operations. Such requirementsand limitations are cost-prohibitive and unacceptable in the industrialmachining environment.

U.S. Pat. No. 5,901,623 (Hong) discloses a cryogenic fluid sprayingchip-breaker which is positioned adjacent the rake face for lifting achip from the rake face after the chip is cut from the workpiece. SeeFIGS. 1B and 1C herein (corresponding to FIGS. 3 and 7B of Hong). Hong'smethod does not provide for uniform cooling of the entire cutting tool,which is desired in the case of hard but brittle tools used inhigh-energy cutting operations.

Moreover, Hong's chip-breaking nozzle requires dedicated tooling andrepositioning if depth of cut and/or feedrate are changed duringcutting. Such requirements and limitations are cost-prohibitive andunacceptable in the industrial machining environment.

U.S. Pat. No. 3,971,114 (Dudley) discloses a cryogenic coolant toolapparatus and method in which the tool is internally routed, theinternal passage is thermally insulated, and the coolant stream isjetted at a precise angle at the interface between the tool edge and theworkpiece so that the chip cutting from the workpiece does not interferewith the stream.

See FIGS. 1D and 1E herein (corresponding to FIGS. 2 and 3A of Dudley).This method also does not provide the desired uniform cooling of hardbut brittle cutting tools used in high-energy cutting operations.Moreover, it requires an involved, dedicated tooling. This requirementis cost-prohibitive and unacceptable in the industrial machiningenvironment.

U.S. Pat. No. 5,103,701 (Lundin, et al.) discloses a method andapparatus for the diamond machining of materials which detrimentallyreact with diamond cutting tools in which hard but brittle tools, whichimproves tool life in cutting operations characterized by power fluxvalues exceeding the common values recommended for conventionalmachining processes by tool manufacturers, tool suppliers, and technicalauthorities recognized within the machining industry.

It is further desired to have an apparatus and a method for cooling suchcutting tools that increases work material cutting speeds and/orproductivity, both of which are limited by the lifetime (and costs) ofcutting tools, inserts, and tips.

It is still further desired to have an apparatus and a method formachining a workpiece which improves safety and environmental conditionsat workplaces by using a cryogenic coolant to cool cutting tools,thereby eliminating conventional, emulsified cutting fluids and/or oilmists.

It is still further desired to have an apparatus and a method formachining a workpiece which improves safety and environmental conditionsat workplaces by minimizing the risks of chip fires, burns and/or chipvapor emissions while using an environmentally acceptable, safe,non-toxic and clean method of cooling cutting tools.

It is still further desired to have an apparatus and a method formachining which reduces production costs by elimination of workpart,workplace, and/or machine cleaning necessitated by the use ofconventional, emulsified cutting fluids and/or oil mists.

It is still further desired to have an apparatus and a method formachining which provides for effective cutting of work materials thatcannot tolerate conventional, emulsified cutting fluids and/or oilmists, such as medical products or powder-metallurgy parts characterizedby open porosity.

It is still further desired to have an apparatus and a method forcooling cutting tools, an apparatus and a method for controlling coolingof cutting tools during cutting operations, and an apparatus and amethod for machining a workpiece, which overcome the difficulties anddisadvantages of the prior art to provide better and more advantageousresults.

BRIEF SUMMARY OF THE INVENTION

Applicants' invention is an apparatus and a method for cooling a cuttingtool, an apparatus and a method for controlling cooling of a cuttingtool during a cutting operation, and an apparatus and a method forcooling a workpiece. Another aspect of the invention is an apparatus anda method for machining a workpiece with a cutting tool using theapparatus and method for cooling the cutting tool and/or the apparatusand method for controlling cooling of the cutting tool. Other aspectsare a workpiece machined by the apparatus and method for machining, andthe recyclable chips removed from the workpiece as a byproduct of theapparatus and method for machining.

A first embodiment of the method for cooling a cutting tool includesmultiple steps. The first step is to provide a supply of a cryogenicfluid. The second step is to deliver a free-expanding stabilized jet ofthe cryogenic fluid to the cutting tool. (“A free-expanding stabilizedjet” is defined and discussed in the Detailed Description of theInvention section below.)

There are several variations of the first embodiment of the method forcooling. In one variation, the cutting tool is engaged in a high-energychip-forming and workpiece-cutting operation. Preferably, at least aportion of the cryogenic fluid is selected from a group consisting ofliquid nitrogen, gaseous nitrogen, liquid argon, gaseous argon andmixtures thereof. In another variation, at least a portion of thefree-expanding stabilized jet of the cryogenic fluid has a temperaturebelow about minus 150 degrees Celsius (−150° C.). In another variation,at least a portion of the free-expanding stabilized jet of the cryogenicfluid has a substantially uniform mass flowrate greater than or equal toabout 0.5 lbs/minute and less than or equal to about 5.0 lbs/minute. Inanother variation, at least a portion of the free-expanding stabilizedjet of the cryogenic fluid has a substantially uniform mass flowratehaving a flow pulse cycle time less than or equal to about 10 seconds.In another variation, the cutting tool has a rake surface and at least aportion of the free-expanding stabilized jet of the cryogenic fluidimpinges on at least a portion of the rake surface. In anothervariation, at least a portion of the cutting tool has a traverse rupturestrength (TRS) value of less than about 3000 MPA. In another variation,the cutting tool has a cutting edge and a means for delivering thefree-expanding stabilized jet of the cryogenic fluid to the cutting toolhas at least one discharge point spaced apart from the cutting edge by adistance greater than or equal to about 0.1 inches and less than about3.0 inches. In a variant of this variation, at least a portion of thecryogenic fluid has a pressure greater than or equal to about 25 psigand less than or equal to about 250 psig during or immediately prior todischarge from the at least one discharge point.

In another embodiment of the method for cooling a cutting tool, in whichthe cutting tool has a cutting edge, there are multiple steps. The firststep is to provide a supply of a cryogenic fluid. The second step is toprovide a nozzle adapted to discharge a jet of the cryogenic fluid. Thenozzle has at least one discharge point spaced apart from the cuttingedge by a distance greater than or equal to about 0.1 inches and lessthan about 3.0 inches. The third step is to deliver a free-expandingstabilized jet of the cryogenic fluid from the discharge point to thecutting tool, wherein the cryogenic fluid has a temperature of aboutminus 150 degrees Celsius (−150° C.) at the discharge point.

Another aspect of the invention is a method for machining a workpiecewith a cutting tool using a method for cooling the cutting tool as inthe first embodiment of the method for cooling. Other aspects are aworkpiece machined by such a method for machining and characterized byan improved surface, and recyclable chips removed from the workpiece asa byproduct of the method for machining the workpiece, the recyclablechips being characterized by an improved purity.

The method for cooling a workpiece involves multiple steps. The firststep is to provide a supply of a cryogenic fluid. The second step is todeliver a free-expanding stabilized jet of the cryogenic fluid to theworkpiece.

A first embodiment of the method for controlling cooling of a cuttingtool during a cutting operation includes multiple steps. The first stepis to provide a supply of a cryogenic fluid. The second step is todeliver a flow of the cryogenic fluid to the cutting tool. The thirdstep is to regulate the flow of the cryogenic fluid to the cutting toolat a substantially uniform mass flowrate, whereby a frost coating ismaintained on at least a portion of the cutting tool duringsubstantially all of the cutting operation in an atmosphere having anambient relative humidity in a range of about 30% to about 75% and anambient temperature in a range of about 10° C. to about 25° C. In onevariation of this embodiment, the cutting tool is engaged in ahigh-energy chip-forming and workpiece-cutting operation.

Another embodiment of the method for controlling cooling of a cuttingtool during a cutting operation includes multiple steps. The first stepis to provide a supply of a cryogenic fluid. The second step is toprovide a nozzle adapted to discharge a flow of the cryogenic fluid, thenozzle having at least one discharge point spaced apart from the cuttingtool. A third step is to deliver a flow of the cryogenic fluid from thedischarge point to the cutting tool. The fourth step is to regulate theflow of the cryogenic fluid to the cutting tool at a substantiallyuniform mass flowrate greater than or equal to about 0.5 lbs/minute andless than or equal to about 5.0 lbs/minute having a flow pulse cycletime less than or equal to about 10 seconds, whereby a frost coating ismaintained on at least a portion of the cutting tool duringsubstantially all of the cutting operation in an atmosphere having anambient relative humidity in a range of about 30% to about 75% and anambient temperature in a range of about 10° C. to about 25° C.

Another aspect of the invention is a method for machining a workpiecewith a cutting tool using a method for controlling cooling of thecutting tool as in the first embodiment of the method for controllingcooling. Other aspects are a workpiece machined by this method formachining and characterized by an improved surface, and the recyclablechips removed from the workpiece as a byproduct of this method formachining, which chips are characterized by an improved purity.

A first embodiment of the apparatus for cooling a cutting tool includes:a supply of a cryogenic fluid; and means for delivering a free-expandingstabilized jet of the cryogenic fluid to the cutting tool.

There are several variations of the first embodiment of the apparatusfor cooling. In one variation, the cutting tool is engaged in ahigh-energy chip-forming and workpiece-cutting operation. Preferably, atleast a portion of the cryogenic fluid is selected from a groupconsisting of liquid nitrogen, gaseous nitrogen, liquid argon, gaseousargon and mixtures thereof. In another variation, at least a portion ofthe free-expanding stabilized jet of the cryogenic fluid has atemperature below about minus 150 degrees Celsius (−150° C.). In anothervariation, at least a portion of the free-expanding stabilized jet ofthe cryogenic fluid has a substantially uniform mass flowrate greaterthan or equal to about 0.5 lbs/minute and less than or equal to about5.0 lbs/minute. In another variation, at least a portion of thefree-expanding stabilized jet of the cryogenic fluid has a substantiallyuniform mass flowrate having a flow pulse cycle time less than or equalto about 10 seconds. In another variation, the cutting tool has a rakesurface and at least a portion of the free-expanding stabilized jet ofthe cryogenic fluid impinges on at least a portion of the rake surface.In another variation, at least a portion of the cutting tool has atraverse rupture strength (TRS) value of less than about 3000 MPa. Inanother variation, the cutting tool has a cutting edge and a means fordelivering the free-expanding stabilized jet of the cryogenic fluid tothe cutting tool has at least one discharge point spaced apart from thecutting edge by a distance greater than or equal to about 0.1 inches andless than about 3.0 inches. In a variant of this variation, at least aportion of the free-expanding stabilized jet of the cryogenic fluid hasa pressure greater than or equal to about 25 psig and less than or equalto about 250 psig during or immediately prior to discharge from the atleast one discharge point.

In another embodiment of the apparatus for cooling a cutting tool, inwhich the cutting tool has a cutting edge, there are several elements.The first element is a supply of a cryogenic fluid. The second elementis a nozzle adapted to discharge a jet of the cryogenic fluid. Thenozzle has at least one discharge point spaced apart from the cuttingedge by a distance greater than or equal to about 0.1 inches and lessthan about 3.0 inches. The third element is a means for delivering afree-expanding stabilized jet of the cryogenic fluid from the dischargepoint to the cutting tool, wherein the cryogenic fluid has a temperatureof about minus 150 degrees Celsius (−150° C.) at the discharge point.

Another aspect of the invention is an apparatus for machining aworkpiece with a cutting tool using an apparatus for cooling the cuttingtool as in the first embodiment of the apparatus. Other aspects are aworkpiece machined by an apparatus for machining and characterized by animproved surface, and recyclable chips removed from the workpiece as abyproduct, the recyclable chips being characterized by an improvedpurity.

The apparatus for cooling a workpiece includes: a supply of a cryogenicfluid; and a means for delivering a free-expanding stabilized jet of thecryogenic fluid to the workpiece.

A first embodiment of the apparatus for controlling cooling of a cuttingtool during a cutting operation includes several elements. The firstelement is a supply of a cryogenic fluid. The second element is a meansfor delivering a flow of the cryogenic fluid to the cutting tool. Thethird element is a means for regulating the flow of the cryogenic fluidto the cutting tool at a substantially uniform mass flow rate, whereby afrost coating is maintained on at least a portion of the cutting toolduring substantially all of the cutting operation in an atmospherehaving an ambient relative humidity in a range of about 30% to about 75%and an ambient temperature in a range of about 10° C. to about 25° C. Inone variation of this embodiment, the cutting tool is engaged in ahigh-energy chip-forming and workpiece-cutting operation.

Another embodiment of the apparatus for controlling cooling of a cuttingtool during a cutting operation includes several elements. The firstelement is a supply of a cryogenic fluid. The second element is a nozzleadapted to discharge a flow of the cryogenic fluid. The nozzle has atleast one discharge point spaced apart from the cutting tool. The thirdelement is a means for delivering a flow of the cryogenic fluid from thedischarge point to the cutting tool. The fourth element is a means forregulating the flow of the cryogenic fluid to the cutting tool at asubstantially uniform mass flowrate greater than or equal to about 0.5lbs/minute and less than or equal to about 5.0 lbs/minute having a flowpulse cycle time less than or equal to about 10 seconds, whereby a frostcoating is maintained on at least a portion of the cutting tool duringsubstantially all of the cutting operation in an atmosphere having anambient relative humidity in a range of about 30% to about 75% and anambient temperature in a range of about 10° C. to about 25° C.

Another aspect of the invention is an apparatus for machining aworkpiece with a cutting tool using a method for controlling cooling ofthe cutting tool as in the first embodiment of the apparatus forcontrolling cooling. Other aspects are a workpiece machined by thisapparatus for machining and characterized by an improved surface, andthe recyclable chips removed from the workpiece as a byproduct, whichchips are characterized by an improved impurity.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described by way of example with reference to theaccompanying drawings, in which:

FIGS. 1A to 1E illustrate various prior art devices used for cryogeniccooling in cutting or machining operations;

FIG. 2A is a schematic illustration of one embodiment of the invention;

FIG. 2B is a schematic illustration of an embodiment of the inventionused in a turning operation;

FIG. 2C is a schematic illustration of an embodiment of the inventionused in a milling operation;

FIG. 3A is a graph illustrating the tool nose temperature over time nosetemperature during high-energy turning of stainless steel 440C;

FIG. 3D is a graph illustrating the effect of the RPM of a cutter onimpact flowrate of pulsing cryo-fluid reaching a cutting insert;

FIG. 4 is a graph illustrating tool life and temperature in high-energycutting of Ti-6Al-4V;

FIG. 5A is a graph illustrating the life of a ceramic composite toolused in a high-energy cutting operation at the speed of 300 ft/minute;

FIG. 5B is a graph illustrating the life of a ceramic composite toolused in a high-energy cutting operation at the speeds of 300 ft/minuteand 400 ft/minute;

FIG. 6 is a graph illustrating the effect of pulsing cryo-fluid jet onthe life of cubic boron nitride under certain conditions; and

FIG. 7 is a graph illustrating the effect of the invention on thechemistry of chips collected for a Ti-6Al-4V work material.

DETAILED DESCRIPTION OF THE INVENTION

The invention addresses fundamental, unresolved needs of the machiningindustry—to produce cleaner parts faster and at less cost, and toimprove environmental and health conditions in manufacturing operations.An important factor in reducing manufacturing costs is to replace slowgrinding operations on hard to machine parts with more cost-effectivecutting operations. The machining industry needs improved methods forhard-turning. Another important but frequently overlooked factor is thecost of tooling and conventional process modifications. The machiningindustry needs machining process improvements that also minimize theextent of the modifications required to existing equipment andprocesses.

The invention is an apparatus and a method for cooling a cutting tool,an insert, a tip, an edge, a blade, or a bit, any of which may be eitherstationary or moving (e.g., rotating, with respect to a workpiece), byusing a free-expanding (unconstrained) stabilized jet of cryogenicfluid. The jet of cryogenic fluid, which may be a single phase gas, asingle phase liquid, or a two-phase combination, preferably is liquidnitrogen, gaseous nitrogen, liquid argon, gaseous argon, and/or mixturesthereof. However, persons skilled in the art will recognize that othercryogenic mixtures of liquids, gases, and solid particles could be usedas the cryogenic fluid.

The free-expanding or unconstrained jet is a stream of cryogenic fluidexpanded from a higher pressure via a nozzle into an unconfinedsurrounding or a space. Due to differences in velocity, density, andtemperature, the resultant shearing forces and mixing eddies lead to theaspiration of surrounding gas(es), such as ambient air. A jet expandingfrom a nozzle located at or above a flat plane, such as rake surface, isfree-expanding, but a jet expanding between two or more fixed planes isnot free-expanding, because the boundary film attachment effect issignificantly enhanced and aspiration of the surrounding gas atmosphereis significantly reduced. (Rake surface is the cutting tool surfaceadjacent the cutting edge which directs the flow of the chip away fromthe workpiece. In the embodiment shown in FIG. 2A, rake surface is thecutting tool surface adjacent the cutting edge which directs the flow ofthe chip 86. The rake surface may be completely flat, chamfered or mayhave a more complex, three-dimensional topography produced by molding oran addition of a plate in order to provide an enhanced control chip flowand/or chip breaking.)

The nozzle for issuing a free-expanding jet may be made of tubingterminating behind, above, or at the rake surface. Alternatively, thenozzle also may be made in the form of a channel drilled in aninsert-holding clamp 80 holding a cutting tool on the back end within atoolholder 82 as shown in FIG. 2A. The nozzle may be formed by anycombination of fixed or adjustable mechanical components attached to aninsert-holding clamp or a toolholder which have channels drilled for thedischarge of the cryogenic fluid from the desired distance at a rakesurface and toward a cutting edge of the rake surface. FIG. 2Billustrates an example of an adjustable-angle nozzle attached to atoolholder. The nozzle exit may be round or flat vertically orhorizontally, converging, straight or diverging. There are no particularlimitations on the nozzle, as long as the nozzle jets a free-expandingjet of cryogenic fluid at the tool from the desired distance in thedesired direction while positioned away from the chip.

FIG. 2A illustrates a preferred embodiment of an apparatus 70 taught bythe invention in which a free-expanding jet of cryogenic fluid 72 isdirected at the surface of a tip 74 of a cutting tool. Cryogenic fluidpasses through a delivery tube 76 and through bore 78 which is drilledthroughout a clamp 80 to form a nozzle. The clamp is attached to atoolholder 82 by a fastening mechanism (not shown). The jet of cryogenicfluid expands from the nozzle onto the tip 74 of a cutting insert 84. Ina most preferred mode of operation, the free-expanding jet terminates atthe surface of the tip of the cutting insert. Alternatively, thefree-expanding jet may be allowed to expand further away to reach thechip 86 evolving from the workpiece as well as the surface of theworkpiece around the chip and the tool/workpiece contact zone.

The embodiment shown in FIG. 2A minimizes the extent of modificationsneeded on a standard machining tool set-up to practice the presentinvention. The cryogenic fluid jetting nozzle is incorporated into ametal clamp 80 commonly used for holding the cutting ?O inserts 84 inwork position, which cutting inserts may be made of a brittle material.The exit of the nozzle and the front part of the clamp are located awayfrom the chip 86 evolving from the workpiece 88 during cutting, and arenever in continuous contact with the chip and do not participate in thechip breaking operation.

The illustration in FIG. 2A shows the direction 90 of rotation (measuredin RPM) of the workpiece 88, the depth of cut (DOC) 92, the feed rate(F=undeformed chip thickness) 94, and the cutting power 96.

FIGS. 2B and 2C illustrate a preferred mode of jet application inturning and milling operations. The jet of cryogenic fluid 72 impingesdirectly at the target tool. For the turning application (FIG. 2B), thecryogenic fluid enters delivery tube 76 and is discharged from thenozzle assembly 98, which is an adjustable-angle nozzle. Afree-expanding stabilized jet of cryogenic fluid is transmitted from thenozzle assembly to the tool nose of the cutting tool insert 84. Theaxial length 100 of the jet from the nozzle exit to the tool nose is acritical feature of the invention, as discussed herein. The arrows 102indicate entrainment of ambient air by the jet.

In the milling operation shown in FIG. 2C, the free-expanding stabilizedjet of cryogenic fluid 72 is transmitted from a nozzle at the end of thedelivery tube 76. The distance between the nozzle and the tool 104 mustbe less than three inches. The arrows 102 indicate entrained ambientair. The tool rotates in the direction shown by the arrow 106 as theworkpiece 88 moves in the direction shown by the arrow 108. The figureillustrates a depth of cut 92, a width of cut 110, and the chips 86′formed by the milling process.

Cryogenic nitrogen and/or argon fluids (in liquid or gaseous phase) arepreferred because these fluids are inert, non-toxic, non-corrosive,acceptable environmentally, and can be made sufficiently cold at theexit of the nozzle to refrigerate a remote target, such as a cuttingtool, if jetted at the target from a distance. The boiling points ofliquid nitrogen, liquid argon, and several other cryogenic fluids scalewith their delivery pressure to reach the following minimum if expandedinto a 1 atmosphere pressure environment:

-   -   liquid N₂=−196° C.=−320° F.    -   liquid Ar=−186° C.=−302° F.    -   liquid CO₂=−79° C.=−110° F. (sublimation point)    -   chlorofluorocarbon Freon-12 CCl₂F₂=−30° C.=−22° F.

An expanding jet tends to entrain a large quantity of ambient gas, suchas room temperature air in typical machining operations. The entrainmentof room temperature air results in a drastic reduction of refrigerationcapacity of a cryogenic jet within a relatively short distance from anozzle exit. U.S. Pat. No. 5,738,281 (Zurecki et al.) discloses a methodof minimizing this entrainment in the case of isothermal or preheatedgas jets.

However, that patent does not teach about free-expanding, cryogenic jetswhich tend to expand both radially and axially on mixing with warmersurroundings.

Applicants discovered that if a cryogenic fluid is jetted from adistance of 0.1 to 3.0 inches at a target tool surface, has an initialtemperature at the nozzle exit less than minus 150° C. (−150° C.), andhas a flowrate of at least 0.5 lbs/minute, then the jet of cryogenicfluid arriving at the tool surface is sufficiently cold and can,potentially, enhance the life of the tool under high-energy cuttingconditions. Applicants also discovered that if the flowrate of thecryogenic fluid jet exceeds 5.0 lbs/minute (37.8 grams/second),excessive spreading of the jet of cryogenic fluid within the cuttingarea results in a detrimental pre-cooling of workpiece material, atransient effect of hardening the workpiece just upstream of the cuttingedge, leading to a drop in tool life. Applicants also determined thatthe minimum discharge pressure required for effective tool cooling is 25psig (1.7 atm). The maximum pressure (250 psig) is established by thelarge-scale economics of storing and handling cryogenic nitrogen andargon—the most common and cost-effective large tanks holding thesecryogens are rated up to 230 psig and rapidly vent a thermallycompressed and expanding cryogen if the cryogen pressure exceeds 250psig (17 atm). Applicants recognized that in order to meet the economicnecessities of the machining industry, the cryogenic cooling of toolsengaged in high-energy cutting operations should be performed using acryo-fluid stream sourced from a large, “bulk” tank under its owncryogenic vapor pressure. Thus, Applicants optimized their tool coolingprocedure for a maximum discharge pressure of no more than 250 psig. Thedischarge pressure is the pressure measured at the inlet side of thecryogenic fluid jetting nozzle.

The free-expanding jet of cryogenic fluid should be aimed toward therake, nose, and cutting edge of the cutting tool to maximize the coolingeffect. If the use of multiple cryo-jets is desired in a given cuttingoperation due to work material or tool geometry considerations, theprimary cryo-jet characterized by the highest flowrate should be aimedtoward the rake, nose, and cutting edge. Applicants found it surprisingand unexpected that the cryogenic fluid jet impinged at the rake surfacein such a way that the jet does not induce fractures, chipping, orcleavage of hard but brittle tool materials preferred in high-energycutting operations. The advanced, non-metallic tools, as well as otherhard but brittle tools (characterized by a traverse rupture strength ofless than 3 GPa or a fracture toughness of less than 10 MPa m^(0.5))cooled according to Applicants' method lasted longer than the same typeof tools operated dry under high-energy cutting conditions. This findingis contrary to the teachings of the prior art

While the exact reasons for the surprising and unexpected results (whichprovide a substantial improvement over the prior art) are not clear, itappears that these results may be due to a combination of factors.Without wishing to be bound by any particular theory, Applicants believethat these factors include but are not limited to: (1) cryogenichardening of the entire cutting tool material, (2) reduction in thermalexpansion-driven stresses within the entire tool, and (3) reduction inthermal gradients at tool surfaces due to the boundary film effect andthe Leidenfrost phenomenon. The boundary film is a jettingcondition-controlled, semi-stagnant, transient film which “softens” thecryogenic chilling effect and “smoothens” thermal profiles at theimpingement-cooled surface. The Leidenfrost phenomenon occurs to alarger or smaller degree with all liquids sprayed at a target surfacethat is hotter than the boiling point of the liquid. Liquid drops boilabove a hot surface and, thus, the hot surface is screened by a layer ofvapor. In the case of cryogenic liquids, especially if colder than −150°C., all tool surfaces are hot, which means that a typical cryo-liquidjet slides on a boundary film of its vapor without directly wetting thetool. This makes the thermal profile of the impingement-cooled toolsurface smoother and may explain why Applicants' free-expandingcryo-fluid jet is effective in enhancing the life of brittle tools. Inthe case of an oil or water-based cutting fluid, with its boiling pointsignificantly higher than room temperature, boiling occurs only at avery close distance from the perimeter of a chip contact zone at a toolsurface. When the chip changes direction during cutting, or the cuttingtool encounters a sudden cutting interruption, such a conventional fluidspreads over a suddenly exposed, hottest tool surface area where itboils explosively, releasing vapor, microdroplets, and pressure waves.The boundary film thickness, Leidenfrost phenomenon, sudden changes inboiling behavior with a change in temperature difference between jettedliquid and target surface (hydrodynamic instabilities), as well as theimportance of nozzle orientation and flow conditions, have been taughtin many references.⁽³⁾ Applicants believe that their method, practicedwithin the above-described range of cryo-fluid jetting conditions,promotes the desired, thin boundary film and/or Leidenfrost effectswhich, in turn, prevents fracturing of brittle tools while cooling andenhances tool life during high-energy cutting operations.

As shown in FIGS. 2B and 2C, Applicants' method and conditions forfree-expanding jet cooling overcome the fundamental shortcomings of theprior art pertaining to cryogenic machining. Since Applicants' nozzle islocated well behind the area of chip formation and the work-tool contactzone, the feed-rate, depth of cut, and other machining conditions couldbe easily changed during a given operation without a need forreadjusting the nozzle placement or risk of nozzle damage. Thus, themachining industry may practice the invention with minimum costs, nodisruptions, enjoying the operational flexibility that arises from thefact that the nozzle is not attached to a cutting insert or dependant onany particular insert geometry. A key to an effective tool cooling withthe free-expanding jet is the adjustment of the cryo-fluid flowratewithin the range of 0.5 to 5.0 lbs/minute and the supply pressure withinthe range of 25 to 250 psig in order to deliver its refrigerationcapacity from the exit of the remote nozzle to the rake surface.

Applicants found that a time-average cryo-fluid flowrate becomessufficient only when the walls of the cutting tool are frosted duringthe entire cutting operation in spite of the fact that a significantamount of cutting energy, i.e., heat, enters the tool through the hotchip contact area. If the frost line forms during cutting near thecutting edge and contact zone on the side walls and rake which movesback toward the other end of the tool, the cryogenic cooling effect isdiminished, indicating the need for an increase in the time averagedflowrate and/or pressure of the cryo-fluid. Note that under thepreferred conditions, no frost coating is expected to develop inside thespot of the direct impingement of the cryogenic fluid, a moisture-freeproduct of N₂ and/or Ar. Thus, a part of the rake and/or side-wallsurface may be free of frost coating because of a continuous washing bya rapidly expanding and moisture-free cryo-fluid.

An exception to the tool frost-coating rule would occur if cuttingoperations are carried out under very low humidity conditions, e.g., ina controlled atmosphere chamber or in a vacuum where the benefits of theinvention could be achieved without producing a frost coating. Thenormal atmospheric conditions for the tool frosting control are 55%relative humidity (RH) plus or minus 20% and 20° C. temperature plus orminus 5° C. The minimum moisture content for the frosting control is 30%relative humidity at a temperature of at least 10° C.

Applicants also developed a diagnostic technique for controlling thehigh-energy cutting operation carried out according to the invention andinvolving observation of dynamic effects at the tool-workpiece interfacewhich may change during any particular operation as the tool wears orcutting conditions are changed. First, if the chip or work surface justbelow the cutting edge is bright red, or appears to melt, or burn, theflowrate and/or pressure of the cryo-fluid should be increased. Second,if the tool nose or the perimeter of the chip contact area on the rakesurface is cherry-red, there is no need to increase the flowrate and/orpressure of the cryo-fluid unless the frosted coating on the tool startsto shrink. Third, if the tool nose or the perimeter of the chip contactarea on the rake is intensely bright red, the flowrate and/or pressureof the cryogenic fluid should be increased regardless of the conditionof the frosted coating on tool surface. An occasional local temperatureincrease at the work/tool contact area may indicate geometric orcompositional inhomogeneities of the work material, and can be easilyquenched by increasing the flowrate of the cryogenic fluid within theprescribed range of 0.5 to 5.0 lbs/minute to the point at which thewhole contact zone, not just the tool surface, is cooled in a directcryogenic fluid impingement mode.

A cutting tool cryo-cooling operation carried out according to the aboveguidelines will provide for improved results. It was surprising andunexpected to Applicants that their cryogenic fluid cooling methodresulted in an improved fracture resistance of brittle cutting toolsduring cutting, an improved life of tools engaged in high-energycutting, and improved surface of machined work material, mirror-cleanchips, and a practical, low-cost process control method based on visualobservation of the frost coating and the tool nose during cutting. Theseimproved results were surprising and unexpected to Applicants and wouldbe surprising and unexpected to other persons skilled in the art.

One of the basic technical problems with the transfer of compressedcryogenic fluids and discharging of free-expanding jets of cryogenicfluid is a tendency for pulsing and boiling flow instabilities,especially if flowrates fall below 1.1 lbs/minute, which overlaps thelower range of flowrates required by Applicants' method. Since thepulsing flow problem would significantly limit industrial applicationsof cryogenic fluids, a number of more or less effective flow-stabilizingsystems have been developed which include a combination of cryogenicsubcooling below the temperature of equilibrium vapor and venting vaporformed in transfer lines.

Some more recent examples of such flow-stabilizing systems are disclosedin U.S. Pat. No. 5,392,608 (Lee), U.S. Pat. No. 5,123,250 (Maric), U.S.Pat. No. 4,716,738 (Tatge), U.S. Pat. No. 4,510,760 (Wieland), and U.S.Pat. No. 4,296,610 (Davis). A method of stabilizing a low-flowratecryogenic fluid flow in industrial machining and cutting applicationswas presented by Zurecki and Harriott, “Industrial Systems for CostEffective Machining of Metals Using an Environmentally Friendly LiquidNitrogen Coolant”, Aerospace Manufacturing Technology Conference, Jun.2-4, 1998, Long Beach, Calif., Session MP5C, Machining and MachiningProcesses—Coolants and Process Safety, Paper No. 981,865, and by Zureckiet al., “Dry Machining of Metals With Liquid Nitrogen”, the 3^(rd)International Machining & Grinding '99 Conference and Exposition,October 4-7, Cincinnati, Ohio, 1999. Since the described systems vary incost and complexity, it is important to identify the key featuresdetermining the effectiveness of a given cryo-fluid flow stabilizingsystem in high-energy cutting operations.

Applicants discovered that the cycle time of pulsing flow is criticalfor an effective free-expanding of a cryogenic fluid jet and aneffective tool cooling under high-energy conditions.

FIG. 3A shows a change in the temperature of a hard WC-Co cutting insertduring high-energy turning of stainless steel (grade 440C) withoutcooling, i.e., dry, with conventional emulsion flood cooling, and withliquid nitrogen cooling applied according to the present invention. Thehigh-energy turning was at a depth of cut of 0.025 inches and a feedrateof 0.010 in/rev. using a carbide insert tool described as follows:CNMG-432, PVD-coated, ISO: M01-M20 (K01-K20), Industry Code: C-3. Thecryogenic fluid jet is turned on a few seconds before the cutting toolbegins cutting, i.e., contacting the workpiece and making chips. Such a“cooldown” time is sufficient to pre-quench the most typical tools orinserts to cryogenic temperatures required to practice the invention.The cryogenic stream used in this test was stabilized using a slightsubcooling, and the resulting jet was steady, with no perceptiblepulsation intervals or flowrate amplitude changes.

Based on experiments with cryo-fluid cooling of cutting tools inhigh-energy cutting operations, a jet pulsation amplitude of more than25% of the time-averaged flowrate is both easily detectable andsignificant for the outcome of cooling. A jet pulsing with an amplitudeof less than 25% of its time-averaged flowrate can be considered astable jet for all practical purposes. Temperatures shown in FIG. 3Awere recorded with a micro-thermocouple tip located 1.41 mm behind thecutting edge and 1.41 mm below the rake surface, inside the insert nosenext to the cutting edge. In all three machining operations noted inFIG. 3A, the time delay between the start of the cut and the wave of theheat diffusing from the edge and arriving at the thermocouple tip wasfrom one to two seconds. After this delay, the temperature stabilized atits own characteristic level reflecting the effectiveness of toolcooling: minus 200° F. (−200° F.=−129° C.) for the liquid nitrogen jetcooling, plus 150° F. (+150° F=+65° C.) for the conventional floodcooling, and more than plus 300OF (+300° F.=+149° C.) for the drycutting. The continuous climbing of the temperature in the case of drycutting reflected a progressive heat accumulation and wearing of thecutting tool leading to an increasing cutting power flux entering thetool.

FIG. 3B shows the change overtime in the temperature of the nose of thefront-end of an insert (next to the cutting edge) and in the temperatureof the back corner of the insert during high-energy turning of Ti-6Al-4VELI. The type of insert used in FIG. 3B is the same as in FIG. 3A, butthe cutting conditions are much heavier, and the flowrate of liquidnitrogen applied for cooling is less than required in Applicants'method. The high-energy turning in this case (FIG. 3B) was at a depth ofcut of 0.120 inches, a feed rate of 0.010 in/rev., and a cutting speedof 230 ft./minute. After a few seconds from the start of cutting, thefrost coating on the insert shrinks and starts to completely disappearfrom its walls, while the nose of the front-end heats up to the point ofemitting red light. The deficient cooling and thermal imbalance resultin a rapid wear of this insert.

FIG. 3C shows a magnification of the initial, non-steady-state portionof the temperature plots from FIG. 3A. More specifically, FIG. 3C showsthe correlation between cryo-fluid pulse cycle and tool nose temperaturein high-energy turning of stainless steel (grade 440C). Two differentpulsing flow profiles are superimposed on this graph to show the effectof frequency and phase shift on insert cooling during the first secondsof cutting. If the cryogenic fluid pulse cycle time is short compared tothe 1-2 second delay in the heating of the nose of the frond-end, theinsert material is “unable to sense” the pulsation and behaves as if theinsert material was cooled by a steady jet impacting the tool with atime-averaged flowrate. If a cryogenic fluid pulse cycle time is longcompared to the 1-2 second time delay, the insert material may betemporarily undercooled or overcooled depending on the phase shiftbetween the jet amplitude and the start-up delay interval. The formerresults in a dangerous overshooting of the temperature of the nose ofthe front-end leading to a steep temperature excursion, as shown in FIG.3B, and to a rapid tool wear. Since the synchronization of the jet pulsephase with the delay interval is impractical and difficult underindustrial cutting conditions, the best practical solution is to use acryo-fluid jet that does not pulse at all or has been stabilized enoughto pulse with the cycle time shorter than the start-up delay of a giventool.

Table 4 below details the high-energy cutting conditions used during thetests plotted in FIGS. 3A, 3B and 3C. TABLE 4 EVALUATION OF MACHININGCONDITIONS IN TEST EXAMPLES PRESENTED Cutting Speed Actual Work Assumed:Unit Feed- Recommended Cutting Material Unit Energy Depth rate, forSelected Speed Removal Power in Cutting Total Power Work of cut, FFeedrate, Med. Used in Rate, in Cutting Ec Power, Flux, Work MaterialTool Type and DOC [inch/ Value, Vc-x Test, Vc MRR Pc [hp/ [Joules/ P Pf[kW/ Material Hardness Material [inches] rev] [feet/min] [feet/min][in3/min] in3/min] mm3] [hp] mm2] Stainless steel, 25 HRC indexablecarbide, 0.025 0.010 410 — 1.2 1.0 2.7 1.2 5.7 440C grade C-3, SandvikGC1015-1025 Stainless steel, 25 HRC indexable carbide, 0.025 0.010 — 6251.9 1.0 2.7 1.9 8.7 440C grade C-3, Sandvik GC1015-1025 Stainless steel,25 HRC indexable carbide, 0.025 0.010 — 1015 3.0 1.0 2.7 3.0 14.1 440Cgrade C-3, Sandvik GC1015-1025 Ti-6Al-4V 32 HRC indexable carbide, 0.1200.010 165 — 2.4 1.8 4.9 4.3 4.1 ELI alloy C-3, Sandvik GC1015-1025Ti-6Al-4V 32 HRC indexable carbide, 0.120 0.010 — 230 3.3 1.8 4.9 6.05.7 ELI alloy C-3, Sandvik GC1015-1025 Ti-6Al-4V 32 HRC indexablecarbide, 0.030 0.008 150 — 0.4 1.8 4.9 0.8 3.7 ELI alloy C-3, SandvikGC1015-1025 Ti-6Al-4V 32 HRC indexable carbide, 0.030 0.008 — 750 2.21.8 4.9 3.9 18.7 ELI alloy C-3, Sandvik GC1015-1025 Hardened tool 62 HRCindexable ceramic 0.020 0.005 365 — 0.4 3.8 10.2 1.6 19.0 steel, A2-composite Al2O3- grade SiCw, Sandvik CC670 Hardened tool 62 HRCindexable ceramic 0.020 0.005 — 300 0.4 3.8 10.2 1.4 15.6 steel, A2-composite Al2O3- grade SiCw, Sandvik CC670 Hardened tool 62 HRCindexable ceramic 0.020 0.005 — 400 0.5 3.8 10.2 1.8 20.8 steel, A2-composite grade Al2O3-SiCw, Sandvik CC670 Hardened tool 62 HRC indexablePCBN 0.020 0.004 325 — 0.3 3.8 10.2 1.2 16.9 steel, A2- (low-contentgrade CBN), Sumitomo BN300 Hardened tool 62 HRC indexable PCBN 0.0200.004 — 500 0.5 3.8 10.2 1.8 26.0 steel, A2- (low-content grade CBN),Sumitomo BN300Power Flux = Total Power/DOC/FNOTES:CUTTING POWER, POWER FLUX, AND VELOCITY INDEX ARE ESTIMANTED FROM DATAIN TABLE 1.REFERENCES FOR MACHINING CONDITIONS - SANDVIK COROMANT AND SUMITOMO

FIG. 30 illustrates the effect of pulse jet cooling in the case of arotating tool like the milling cutter illustrated in FIG. 3B. Morespecifically, FIG. 3D shows the effect of the RPM of a cutter on impactflowrate of the pulsing cryogenic fluid reaching the cutting insertunder the following conditions: 8.0 seconds cryo-fluid jet cycle timeand 60 RPM cutter, 0.4 radian phase shift between jet and cutter,average jet flowrate—3.0 lbs/min., jet flowrate deviation=±50%, andaverage impact flowrate=1.5 lbs/minute. Since industrial mill cuttersoperate at high rotational speeds (rpm) with the rotational frequencytypically ranging from 1 Hz to 700 Hz, the pulsing jet flow cycle timeof about 8 seconds (0.125 Hz) is sliced into short sections which are“invisible” to the rotating cutting edge. In effect, the tool behaves asif it was cooled by the cryo-fluid jet that pulses at its original,“low” frequency but impacts the tool with the flowrate reduced by theeffect of rpm superimposed on the lower, time-averaged flowrate of thepulsing jet. This drop in the impact flowrate of the cryo-fluid could becompensated for by increasing the average discharge flowrate of the jetat the nozzle. The practical significance of this example is that noflowrate adjustment could compensate for an excessively long jet pulsingcycle time. Based on the available data, Applicants believe that thereexists a limiting value for the jet pulse cycle time (or frequency) andthat a non-steady cryogenic fluid jet which pulses slower than thelimiting value would be an ineffective coolant for high-energy cuttingoperations regardless of the time-average flowrate.

The time interval of one to two seconds required to reach a steady-statecondition within the front cutting portion of the hard WC-6Co carbideinsert, as shown in FIGS. 3A and 3B, is in line with the experimentaland numerical determination of J. Lin et al., “Estimation of CuttingTemperature in High Speed Machining”, Trans. of the ASME, Vol. 114, July1992, pp. 289-296. Its value is the limiting pulse cycle time valuerequired for an effective cryogenic fluid cooling of, specifically,harder grades of carbide tools engaged in high-energy cuttingoperations. Since its value scales with the thermal diffusivity of toolmaterial, Applicants evaluated it for a range of hard but brittle toolswhich are preferred in high-energy cutting operations. See Table 5below. TABLE 5 TIME REQUIRED TO REACH STEADYSTATE TEMPERATURE ON TOOLRAKE SURFACE BASED ON TEST DATA FOR WC-Co INSERT⁽⁴⁾ source data:calculated: Tool time to steady-state Specific Thermal Thermal relativeto WC-6Co tool Tool Density heat (Cp) conductivity diffusivity t (toolmaterial)/t (WC-6Co) Material g/cm{circumflex over ( )}3 J/(kg K) W/(mK) m{circumflex over ( )}2/sec. where: L (material) = L (WC-Co) WC-6Co14.7 230 100 3.0E−05 1.0 90% PCBN 3.4 810 100 3.6E−05 0.8 50% PCBN 4.3810 44 1.3E−05 2.3 Sl3N4 3.4 170 40 6.9E−05 0.4 Al203 3.9 770 18 6.0E−064.9 assumed constant values at room temperaturet = L² · ρ · C_(p) · λ⁻¹where:t = time to reach steady-state temperature at the distance L from theundeformed chip imprint at the rake surface of a cutting toolρ = specific density of tool materialC_(p) = specific heat of tool materialλ = thermal conductivity of tool material

Due to a relatively low diffusivity as compared to the carbide tool,Al₂O₃-based and low-content PCBN tools were found to carry out the heatfrom the cutting edge about 2.5 to 5 times slower. Thus, for the one totwo second-long time delay recorded in FIG. 3A, the Al₂O₃ time delaywill range from about 5 to 10 seconds. This is in line with thenumerical estimation of A. Kabala for ceramic inserts, “Heat Transfer inCutting Inserts”, Experimental Stress Analysis 2001, the 39^(th)International Conf., Jun. 4-6, 2001, Tabor, Czech Republic, and sets thelimiting value for the maximum pulse cycle time of 10 seconds. Becauseof very high power fluxes (P_(f)) entering cutting tools through thecontact zone in high-energy cutting operations, as shown in Tables 3 and4, a fluctuation in tool cooling exceeding the limiting pulse cycle timeof 10 seconds would lead to a premature tool failure. Consequently, thefree-expanding cryo-fluid jets used in high-energy cutting should besufficiently stabilized during the transfer from the source tank to thenozzle to pulse at a cycle time of less than 10 seconds whenever thepulse amplitude exceeds 25% of the time-averaged flowrate.

EXAMPLES

FIG. 4 shows an evolution of insert temperature and flank wear during ahigh-energy finish-turning test cutting Ti-6Al-4V with a hard grade ofWC-Co insert. The depth of cut was 0.030 inches at a cutting speed of750 ft./min. and a feed rate of 0.008 in./rev. using the same type ofinsert as that used in FIG. 4A. The life of a tool cooled with acryogenic nitrogen jet applied according to the invention was more thanfour (4) times longer than the life of a tool cooled using aconventional (emulsified) flood coolant.

FIG. 5A shows the life of a ceramic composite tool (Al₂O₃—SiC_(w)) in ahigh-energy cutting operation on A2-steel at a speed of 300 ft/minute, adepth of cut of 0.020 inches, a feedrate of 0.005 inches/rev., and aremoval rate of 0.36 in.³/min. The tool life was evaluated using threecriteria: the maximum flank wear, V_(b max)=0.6 mm; the maximum flank(or DOC notch) wear, V_(b max)=0.7 mm; and the dimensional cutting errorproducing parts 0.004 inches (0.1 mm) larger than required. Fourdifferent cutting methods were used: (1) a conventional emulsion flood,(2) a conventional dry, (3) a cryogenic gas-jet applied according to theinvention, and (4) a cryogenic liquid-jet, also applied according to theinvention. The conditions for (1), (3) and (4) were as follows: (1) auniformly flowing and completely flooding conventional cutting fluid,10% concentration; (3) a stable, non-pulsing gas-phase cryogenic jetflowing at 1.8 lbs/minute, nozzle discharge temperature of minus 150° C.at 7.8 atm (115 psig) pressure; and (4) a stable, non-pulsingliquid-phase cryogenic jet containing a minute fraction of vapor, totalflowrate of 0.9 lbs/minute, nozzle discharge temperature of minus 172°C. at 8.1 atm (120 psig). The results point out that the cryo-fluidcooling applied according to the invention extended tool life over thetwo conventional methods.

FIG. 5B shows the life of the same type of tool in the same type of test(as in FIG. 5A) at two cutting speeds of 300 ft/min. (with a materialremoval rate of 0.36 in³/min) and 400 ft/min. (with a material removalrate of 0.48 in³/min.), where the life in minutes is a composite,averaged from the life measurements according to the same three criteriaas above for FIG. 5A. Again, the cryo-fluid cooling applied according tothe invention enhanced tool life during this hardturning test under bothcutting speeds.

FIG. 6 shows an evolution of flank wear and cutting edge chipping duringhardturning of A2-steel with low-PCBN content (BN-300), brazed-tipinsert tools at the speed of 500 ft./minute, a depth of cut of 0.020inches, a feedrate of 0.004 inches/rev., and a removal rate of 0.48in.³/min. Three cutting conditions were compared: (1) dry turning, (2)cryogenic liquid jet cooled turning, where the jet was insufficientlystabilized and pulsed at the frequency of 6 seconds, and (3) cryogenicliquid jet cooled turning, where the jet was completely stabilized andshowed no pulses or flow instabilities. The conditions for (2) and (3)were as follows: (2) liquid-phase cryogenic jet containing a significantvolumetric fraction of vapor, total flowrate of 2.0 lbs/minute, nozzledischarge temperature of minus 169° C. at 10.2 atm (150 psig) pressure,6-sec. cycle; and (3) liquid-phase cryogenic jet containing aninsignificant volumetric fraction of vapor, total flowrate of 2.0lbs/minute, nozzle discharge temperature of minus 169° C. at 10.2 atm(150 psig) pressure. The ISO tool life criterion of maximum flank wear(V_(bmax)) to 0.6 mm was adopted in this test. The shortest cutting edgelife was noted for the conventional dry cutting. The life with thepulsing jet was longer, but the longest tool life was noted for thestable, non-pulsing jet.

Table 4, which was discussed earlier, details the high-energy cuttingconditions used during the tests plotted in FIGS. 4 to 6 and comparesthe cutting speeds and power fluxes to the respective values recommendedby the manufacturers of the tested inserts.

An additional milling test was carried out to correlate tool frostingand jet pulsing with tool performance in high-energy cutting. Themilling cutter used in this test was a ¾ inch (19.05 mm) diameter, 450helix, 5-flute, high-performance carbide (WC—Co) end-mill, S545-type,made by Niagara Cutter (http://www.niagaracutter.com/techinfo) formaximum metal removal rates during machining of Ti-alloys and otherdifficult to machine materials. The recommended speeds and feeds forthis tool were 90 to 160 ft/minute (27.4 to 48.8 m/minute) and 0.002inches/tooth (0.05 mm/tooth), respectively. The following acceleratedcutting conditions were selected for the conventional milling operationwith this cutter using an emulsified cutting fluid (water with “soluble”lubricant): cutting speed—178 ft/minute, rotational speed—907 rpm, feedper tooth—0.003 inches, table feed—13.6 inches/minute, width ofcut—0.080 inches, axial depth of cut—1.000 inches, material removalrate—1.09 in³/minute. Under these cutting conditions, all 5 cutter edgeswere terminally worn after removing of 13.1 in³ of a Ti-6Al-4V workpiececharacterized by a hardness of 36 HRC.

In a comparative test, a liquid nitrogen jet discharged from a pressureof 80 psig at the time-averaged flowrate of 2 lbs/minute was directed atthe cutter from the distance of 0.5 inches between the exit of a remotenozzle and the corners of the flutes of the end-mill as shown in FIG.3B. As a result, the jet impinged on all five flutes and rake surfacesof the cutter. Initially, the jet flow was delivered via an insulatedline from a saturated liquid nitrogen cylinder in an unstabilizedcondition, and the jet pulse cycle was found to be about 15 seconds.During the pulse cycle, the low flowrate was estimated at 0.75lbs/minute, and the high flowrate at 3.25 lbs/minute. It was observedthat the cutter could not develop a white frost coating at the surfaceswhich were unwetted by the impacting cryo-fluid jet for at least aminute after start-up, and once that coating was established, it wasunstable, appearing and disappearing, following the jet pulse cycle withsome delay. The life of the tool cooled with this non-stable jet andtested with the conditions used above was comparable to that of theconventionally cooled tool.

In another comparative test, the liquid nitrogen flowrate was stabilizedusing an upstream, liquid nitrogen subcooling system, so that no jetpulsation could be visually detected. The milling operation was repeatedusing progressively increasing cutting speeds.

It was observed that the frost coating was stable throughout the entireoperation. When the cutting speed, table feedrate, rpm, and materialremoval rate were increased by 60% over the

The results show that the chips produced during the cryo-fluid cuttingcan be more easily recycled than in the case of the conventional cuttingmethods. This is a significant economic benefit in the machiningindustry, especially in the case of expensive and reactive titanium,tantalum and superalloy work parts, since the purification of thesematerials is extremely difficult and expensive. More importantly, thelower contamination of the chips collected indicates a correspondinglylower contamination of the work material, which is desired from thestandpoint of (1) part stress distribution, (2) corrosion resistance,and (3) post-machining processability. It is known that the surface ofmetallic parts characterized by reduced oxygen, carbon, and hydrogencontamination would be more resistant to fatigue cracking in service,less brittle, and more corrosion resistant. Thus, the use of Applicants'free-expanding stabilized cryo-fluid jet cooling method brings about twoadditional economic benefits to the machining industry—improvedproperties of parts produced and more valuable, recyclable chips.

Applicants discovered that if a cutting tool insert is cooled with afree-expanding, cryogenic fluid jet discharged from a remote nozzlelocated away from the cutting zone, the inherent flow instabilities orpulsation of such a jet may unexpectedly interact with the cuttingprocess, affect insert cooling during operation, and reduce its life.Applicants established and optimized cryo-fluid jetting flowrate andstabilizing conditions in order to minimize this problem. None of thesefindings and inventive techniques could be anticipated from the priorart.

With the free-expanding cryo-fluid jet, stabilized according to themethod outlined above, Applicants tried to use the stabilized jet forcooling of hard but brittle tools preferred in high-energy cuttingoperations, such as a high-speed machining, hardturning, or cutting ofdifficult to machine materials in order to enhance tool life underdemanding machining conditions. Unexpectedly, the remote and stabilizedjet cooling resulted in the enhancement of tool life even in the case ofthose tools which, according to prior art and machining publications,should not be cooled with conventional coolants in order to preventbrittle fracturing.

Although illustrated and described herein with reference to certainspecific embodiments, the present invention is nevertheless not intendedto be limited to the details shown. Rather, various modifications may bemade in the details within the scope and range of equivalents of theclaims and without departing from the spirit of the invention.

Kennametal's web page: http://www.kennametal.com/metalworking/htmlspecialty/properties%20 chart.pdf.

1. A method for cooling a cutting tool, comprising the steps of:providing a supply of a cryogenic fluid; and delivering a free-expandingstabilized jet of the cryogenic fluid to the cutting tool.
 2. A methodas in claim 1, wherein the cutting tool has a cutting edge and wherein ameans for delivering the free-expanding stabilized jet of the cryogenicfluid to the cutting tool has at least one discharge point spaced apartfrom the cutting edge by a distance greater than or equal to about 0.1inches and less than about 3.0 inches.
 3. A method as in claim 1,wherein at least a portion of the free-expanding stabilized jet of thecryogenic fluid has a temperature below about minus 150 degrees Celsius(−150° C.).
 4. A method as in claim 2, wherein at least a portion of thecryogenic fluid has a pressure greater than or equal to about 25 psigand less than or equal to about 250 psig during or immediately prior todischarge from the at least one discharge point.
 5. A method as in claim1, wherein at least a portion of the free-expanding stabilized jet ofthe cryogenic fluid has a substantially uniform mass flowrate greaterthan or equal to about 0.5 lbs/minute and less than or equal to about5.0 lbs/minute.
 6. A method as in claim 1, wherein at least a portion ofthe free-expanding stabilized jet of the cryogenic fluid has asubstantially uniform mass flowrate having a flow pulse cycle time lessthan or equal to about 10 seconds.
 7. A method as in claim 1, whereinthe cutting tool has a rake surface and at least a portion of thefree-expanding stabilized jet of the cryogenic fluid impinges on atleast a portion of the rake surface.
 8. A method as in claim 1, whereinat least a portion of the cryogenic fluid is selected from a groupconsisting of liquid nitrogen, gaseous nitrogen, liquid argon, gaseousargon and mixtures thereof.
 9. A method as in claim 1, wherein at leasta portion of the cutting tool has a traverse rupture strength (TRS)value of less than about 3000 MPa.
 10. A method as in claim 1, whereinthe cutting tool is engaged in a high-energy chip-forming andworkpiece-cutting operation.
 11. A method for machining a workpiece witha cutting tool using a method for cooling the cutting tool as inclaim
 1. 12. A workpiece machined by a method as in claim 11 andcharacterized by an improved surface.
 13. Recyclable chips obtained as abyproduct of a method as in claim 11 and characterized by an improvedpurity.
 14. A method for cooling a workpiece, comprising the steps of:providing a supply of a cryogenic fluid; and delivering a free-expandingstabilized jet of the cryogenic fluid to the workpiece.
 15. A method forcontrolling cooling of a cutting tool during a cutting operation,comprising the steps of: providing a supply of a cryogenic fluid;delivering a flow of the cryogenic fluid to the cutting tool; andregulating the flow of the cryogenic fluid to the cutting tool at asubstantially uniform mass flowrate, whereby a frost coating ismaintained on at least a portion of the cutting tool duringsubstantially all of the cutting operation in an atmosphere having anambient relative humidity in a range of about 30% to about 75% and anambient temperature in a range of about 10° C. to about 25° C.
 16. Amethod as in claim 15, wherein the cutting tool is engaged in ahigh-energy chip-forming and workpiece-cutting operation.
 17. A methodfor machining a workpiece with a cutting tool using a method forcontrolling cooling of the cutting tool as in claim
 15. 18. A workpiecemachined by a method as in claim 17 and characterized by an improvedsurface.
 19. Recyclable chips obtained as a byproduct of a method as inclaim 17 and characterized by an improved purity.
 20. A method forcooling a cutting tool having a cutting edge, comprising the steps of:providing a supply of a cryogenic fluid; providing a nozzle adapted todischarge a jet of the cryogenic fluid, said nozzle having at least onedischarge point spaced apart from the cutting edge by a distance greaterthan or equal to about 0.1 inches and less than about 3.0 inches; anddelivering a free-expanding stabilized jet of the cryogenic fluid fromthe discharge point to the cutting tool, wherein the cryogenic fluid hasa temperature of about minus 150 degrees Celsius (−150° C.) at thedischarge point.
 21. A method for controlling cooling of a cutting toolduring a cutting operation, comprising the steps of: providing a supplyof a cryogenic fluid; providing a nozzle adapted to discharge a flow ofthe cryogenic fluid, said nozzle having at least one discharge pointspaced apart from the cutting tool; delivering a flow of the cryogenicfluid from the discharge point to the cutting tool; and regulating theflow of the cryogenic fluid to the cutting tool at a substantiallyuniform mass flowrate greater than or equal to about 0.5 lbs/minute andless than or equal to about 5.0 lbs/minute having a flow pulse cycletime less than or equal to about 10 seconds, whereby a frost coating ismaintained on at least a portion of the cutting tool duringsubstantially all of the cutting operation in an atmosphere having anambient relative humidity in a range of about 30% to about 75% and anambient temperature in a range of D about 10° C. to about 25° C.
 22. Anapparatus for cooling a cutting tool, comprising: a supply of acryogenic fluid; and means for delivering a free-expanding stabilizedjet of the cryogenic fluid to the cutting tool.
 23. An apparatus as inclaim 22, wherein the cutting tool has a cutting edge and wherein themeans for delivering the free-expanding stabilized jet of the cryogenicfluid to the cutting tool has at least one discharge point spaced apartfrom the cutting edge by a distance greater than or equal to about 0.1inches and less than about 3.0 inches.
 24. An apparatus as in claim 22,wherein at least a portion of the free-expanding stabilized jet of thecryogenic fluid has a temperature below about minus 150 degrees Celsius(−150° C.).
 25. An apparatus as in claim 23, wherein at least a portionof the free-expanding stabilized jet of the cryogenic fluid has apressure greater than or equal to about 25 psig and less than or equalto about 250 psig during or immediately prior to discharge from the atleast one discharge point.
 26. An apparatus as in claim 22, wherein atleast a portion of the free-expanding stabilized jet of the cryogenicfluid has a substantially uniform mass flowrate greater than or equal toabout 0.5 lbs/minute and less than or equal to about 5.0 lbs/minute. 27.An apparatus as in claim 22, wherein at least a portion of thefree-expanding stabilized jet of the cryogenic fluid has a substantiallyuniform mass flowrate having a flow pulse cycle time less than or equalto about 10 seconds.
 28. An apparatus as in claim 22, wherein thecutting tool has a rake surface and at least a portion of thefree-expanding stabilized jet of the cryogenic fluid impinges on atleast a portion of the rake surface.
 29. An apparatus as in claim 22,wherein at least a portion of the cryogenic fluid is selected from agroup consisting of liquid nitrogen, gaseous nitrogen, liquid argon,gaseous argon and mixtures thereof.
 30. An apparatus as in claim 22,wherein at least a portion of the cutting tool has a traverse rupturestrength (TRS) value of less than about 3000 MPa.
 31. An apparatus as inclaim 22, wherein the cutting tool is engaged in a high-energychip-forming and workpiece-cutting operation.
 32. An apparatus formachining a workpiece with a cutting tool using an apparatus for coolingthe cutting tool as in claim
 22. 33. A workpiece machined by anapparatus as in claim 32 and characterized by an improved surface. 34.Recyclable chips removed from a workpiece by an apparatus as in claim 32and characterized by an improved purity.
 35. An apparatus for cooling aworkpiece, comprising: a supply of a cryogenic fluid; and means fordelivering a free-expanding stabilized jet of the cryogenic fluid to theworkpiece.
 36. An apparatus for controlling cooling of a cutting toolduring a cutting operation, comprising: a supply of a cryogenic fluid;means for delivering a flow of the cryogenic fluid to the cutting tool;and means for regulating the flow of the cryogenic fluid to the cuttingtool at a substantially uniform mass flowrate, whereby a frost coatingis maintained on at least a portion of the cutting tool duringsubstantially all of the cutting operation in an atmosphere having anambient relative humidity in a range of about 30% to about 75% and anambient temperature in a range of about 10° C. to about 25° C.
 37. Anapparatus as in claim 36, wherein the cutting tool is engaged in ahigh-energy chip-forming and workpiece-cutting operation.
 38. Anapparatus for machining a workpiece with a cutting tool using a methodfor controlling cooling of the cutting tool as in claim
 36. 39. Aworkpiece machined by an apparatus as in claim 38 and characterized byan improved surface.
 40. Recyclable chips removed from a workpiece by anapparatus as in claim 38 and characterized by an improved purity.
 41. Anapparatus for cooling a cutting tool having a cutting edge, comprising:a supply of a cryogenic fluid; a nozzle adapted to discharge a jet ofthe cryogenic fluid, said nozzle having at least one discharge pointspaced apart from the cutting edge by a distance greater than or equalto about 0.1 inches and less than about 3.0 inches; and means fordelivering a free-expanding stabilized jet of the cryogenic fluid fromthe discharge point to the cutting tool, wherein the cryogenic fluid hasa temperature of about minus 150 degrees Celsius (−150° C.) at thedischarge point.
 42. An apparatus for controlling cooling of a cuttingtool during a cutting operation, comprising: a supply of a cryogenicfluid; a nozzle adapted to discharge a flow of the cryogenic fluid, saidnozzle having at least one discharge point spaced apart from the cuttingtool; means for delivering a flow of the cryogenic fluid from thedischarge point to the cutting tool; and means for regulating the flowof the cryogenic fluid to the cutting tool at a substantially uniformmass flowrate greater than or equal to about 0.5 lbs/minute and lessthan or equal to about 5.0 lbs/minute having a flow pulse cycle timeless than or equal to about 10 seconds, whereby a frost coating ismaintained on at least a portion of the cutting tool duringsubstantially all of the cutting operation in an atmosphere having anambient relative humidity in a range of about 30% to about 75% and anambient temperature in a range of about 10° C. to about 25° C.